At least since the advent of overhead valve engines, emission type oil pumps have been employed to draw lubricant from the crankcase sump and distribute it under pressure to various of the internal engine parts. In some instances, the splash system is used wherein oil sprayed on a rotating crank and camshaft is squeezed up through oil galleys and push rods to the top of the engine where they lubricate rocker arms and valves and trickle back down into the sump. In other instances, oil is distributed directly to vital parts, but in either event, the oil pump may generate more than 80 PSI for distribution of the lubricant throughout the engine.
A typical arrangement includes a gear pump enclosed in a casing which is either immersed in the crankcase sump, or has an intake screen immersed in the sump for drawing oil into the pump where it is squeezed between rotating gears typically driven by the distributor shaft and discharged upwardly under pressure to lubricate the engine parts.
In some engine applications, particularly in high performance engines of the wet-sump variety, regulation of the discharge pressure is a highly desirable feature. Several such devices for regulating discharge pressure from the pump have been employed in the past and the typical method of accomplishing pressure regulation is to divert a portion of the discharge flow back to the crankcase sump, thereby modulating engine discharge pressure at a desired level.